(1) Field of the Invention
The present invention relates to a mobile communication system having a base station and a plurality of mobile stations, and more particularly to a mobile communication system having pagers which provide a two-way data and communication and have various transmission capacities.
(2) Description of the Related Art
Generally, a selective calling radio system, called a paging system, has one base station covering a wide zone and offers selective calling service, for example, to 30,000 or more mobile stations (pagers) by using one radio frequency. Specifically, a base station transmits a selection signal (ID code) and a message in response to calling from a general subscriber. On the other hand, a pager, detecting the ID code of self station among the received signals, tells that the self station is being called by means of a lamp or a buzzer, and displays the message following the ID code on a built-in liquid crystal display, etc.
In such a paging system, a pager cannot be called from a base station when the pager is located in an area where radio waves do not reach (for example, in a subway, a tunnel, an area beyond the service area, etc.), or when pager power is turned off (for example, during a conference, a concert, etc.). However, the base station cannot know that the pager is in a condition where the calling is impossible. This is because the communication from the base station to the pager is a one-way communication.
For this reason, a pager which allows a two-way communication and has an individual transmission capacity has been developed. When a pager of this type is called from a base station, the pager transmits the ID code of the self station to the base station so that it tells that it has surely received the call.
However, the conventional pager which allows two-way data communication requires a considerable amount of electric power for transmitting the ID code to the base station, and therefore requires a large-scale circuit configuration of the transmission section. These requirements are contrary to the advantages of pagers where the battery can be used for a long time and the portability is excellent in terms of shape and weight. Therefore, the solution to these problems has been demanded.